I am a beginner too with OMV and linux. I had same problem and fixed it with this post.
I advice to read the post where it's explained with further detail. Anyway, this is basically what I did:
You have to press esc during grub boot to enter in grub setup, then press e to edit the boot script, and then carefully edit your boot partition in the line beginning with linux. In my case I changed sda1 to sdb1 (it is the partition where the system is installed) saved and boot and voilá it worked.
To fix it permanently you have to use update-grub from command line once you've managed to boot. This way I think a unique id is assigned to each disk and no a random letter that can be confusing for the system specially using external usb.
Hope this helps.